Abstract:

A pneumatic hammer mechanism is disclosed. The hammer mechanism includes:
a flying mass; an impact surface which limits a movement of the flying
mass along the impact axis in the impact direction; a hammer piston which
limits a movement of the flying mass opposite from the impact direction;
a pneumatic chamber between the flying mass and hammer piston; and a
drive for periodically moving the hammer piston with a stroke along the
impact axis. The following inequality applies for the mass (m2) of
the flying mass, a cross-sectional area (A) of the pneumatic chamber, the
maximum length (L) of the pneumatic chamber, the stroke (H) of the hammer
piston and an impact coefficient (q), if the hammer mechanism has an
impact frequency (f) during percussive operation:
L κ 2 ( L - H ) κ κ L - H + (
L κ 2 ( L - H ) κ - 1 ) 1 - q q N 2
π H ≧ ! m 2 A p 0 N 2 f 2
##EQU00001## where the parameter N is at least 4, po designates
the ambient pressure and κ the isentropic coefficient of gas in the
pneumatic chamber.

Claims:

1. A pneumatic hammer mechanism, comprising:a flying mass which is movable
along an impact axis;an impact surface which limits a movement of the
flying mass along the impact axis in an impact direction;a hammer piston
which limits the movement of the flying mass along the impact axis
opposite from the impact direction;a pneumatic chamber disposed between
the flying mass and the hammer piston; anda drive for periodically moving
the hammer piston with a stroke along the impact axis, wherein the flying
mass is excited to a periodic movement between the impact surface and a
minimum approach of the hammer piston;wherein a mass (m2) of the
flying mass, a cross-sectional area (A) of the pneumatic chamber, a
maximum length (L) of the pneumatic chamber, the stroke (H) of the hammer
piston, and an impact coefficient (q) fulfill a following inequality, if
the hammer mechanism has an impact frequency (f) during percussive
operation: L κ 2 ( L - H ) κ κ L - H
+ ( L κ 2 ( L - H ) κ - 1 ) 1 - q
q N 2 π H ≧ ! m 2 A p 0 N
2 f 2 ##EQU00007## wherein N is at least 4, po designates an
ambient pressure and κ an isentropic coefficient of gas in the
pneumatic chamber.

2. The pneumatic hammer mechanism according to claim 1, wherein a length
ratio of the maximum length to the stroke of the hammer piston is 1.55.

3. The pneumatic hammer mechanism according to claim 1, wherein if the
mass of the flying mass is greater than 400 g a length ratio of the
maximum length to the stroke of the hammer piston is less than 1.55 and
if the mass of the flying mass is less than 400 g the length ratio is
less than 1.40.

4. The pneumatic hammer mechanism according to claim 1, wherein if a ratio
m1/m2 of a mass (m1) of a snap die to the mass (m2)
of the flying mass is less than 1.2 a length ratio of the maximum length
to the stroke of the hammer piston is less than 1.40.

5. The pneumatic hammer mechanism according to claim 4, wherein the impact
coefficient (q) is 0.22 if the ratio m1/m2 of the mass
(m1) of the snap die to the mass (m2) of the flying mass is
greater than 1.2 and otherwise the impact coefficient (q) is 0.12.

6. The pneumatic hammer mechanism according to claim 4, wherein N is
greater than 5.

Description:

[0001]This application claims the priority of European Patent Document No.
09100088.5, filed Jan. 30, 2009, the disclosure of which is expressly
incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002]The present invention relates to a pneumatic hammer mechanism, in
particular an electrically driven, pneumatic hammer mechanism, for a
power tool, in particular a hand power tool, e.g., a chipping hammer.

[0003]An electrically operated chipping hammer having a pneumatic hammer
mechanism is known from European Patent Document No. EP 1 779 980 A2
among others. A schematic representation of its hammer mechanism 501 from
FIG. 6 is incorporated as FIG. 1.

[0004]A flying mass 569 is arranged in a piston cylinder 530 between a
hammer piston 520 and an end piece of a tool 599. The flying mass 569 and
the hammer piston 520 make an airtight seal with a wall of the piston
cylinder so that a sealed airtight chamber 580 is formed between the
flying mass 569 and the hammer piston 520. The chamber 580 will be called
pneumatic chamber 580 in the following.

[0005]The hammer piston 520 moves periodically in a reciprocating manner
in the piston cylinder 530, driven by a gear wheel 522, 523, 531. The
flying mass 569 is also excited to move periodically between the hammer
piston 520 and the end piece of the tool 599 based on its coupling to the
hammer piston 520 by means of the pneumatic chamber 580.

[0006]FIG. 2 schematically shows the progression of movement of the hammer
piston 520 and flying mass 569 over time t; the progression among other
things is also depicted in FIG. 13A of EP 1 779 980 A2. The local axis x
indicates the distance from the end piece of the tool 599. When the
hammer piston 520 moves at its greatest velocity in the direction of the
tool 599 (at small x values), the hammer piston 520 and the flying mass
569 come as close as possible. The pneumatic chamber 580 is heavily
compressed in the process and as a result accelerates the flying mass 569
in the direction of the tool 599. After this, the flying mass 569 strikes
undamped the end piece of the tool 599. A portion of the kinetic energy
of the flying mass 569 is transferred in the process to the tool. As with
a partial elastic impact with a heavy impact mate, the flying mass 569
reverses its direction of movement and moves with reduced velocity in the
direction of the hammer piston 520. The stroke H of the hammer piston
520, the angular velocity of the hammer piston 520 and the maximum length
of the pneumatic chamber 580 are coordinated with each other such that
the movement of the flying mass 569, as depicted, is excited resonantly
by the hammer piston 520.

[0007]There is the need to further increase the impact effect of the
chipping hammer without increasing the power consumption of the chipping
hammer in the process. The impact effect of the chipping hammer is
produced essentially from the energy released by an impact in a work
piece. The power consumption is yielded from the product of the energy
released per impact and the impact frequency of the impacts.
Consequently, the impact frequency of the impacts must be reduced.

[0008]The energy released by each impact depends upon the kinetic energy
that the flying mass 569 collects up until impact. The acceleration work
is performed by the hammer piston 520, which increases with increasing
velocity of the hammer piston 520 in the piston cylinder 530. The
velocity of the hammer piston 520 is predetermined by the angular
velocity and the stroke H of the hammer piston 520. Even though
increasing the angular velocity based on the impact frequency of the
impacts that increases with it is not suitable, the stroke H of the
hammer piston 520 can be increased. However, this requires a greater
maximum length of the pneumatic chamber 580 and thus a longer hammer
mechanism in order to guarantee a resonant excitation of the flying mass
569.

[0009]So that a user may hold the chipping hammer ergonomically during
operation, the dimensions of the chipping hammer and thus also of the
hammer mechanism are restricted, however.

[0010]The kinetic energy of the flying mass 569 can also be achieved by
increasing its mass, however, an operator then experiences a greater
recoil during acceleration of the flying mass 569 from the hammer piston
520.

[0011]One objective is making a percussive power tool available that
facilitates an improved impact effect taking ergonomic aspects of into
consideration.

[0012]The hammer mechanism features: a flying mass, which is movable along
an impact axis; an impact surface, which limits a movement of the flying
mass along the impact axis in the impact direction; a hammer piston,
which limits a movement of the flying mass along the impact axis opposite
from the impact direction; a pneumatic chamber between the flying mass
and hammer piston; a drive for periodically moving the hammer piston with
a stroke H along the impact axis, wherein the flying mass is excited to a
periodic movement between the impact surface and a minimum approach of
the hammer piston. In this case, the following inequality applies for the
mass m2 of the flying mass, a cross-sectional area A of the
pneumatic chamber, the maximum length L of the pneumatic chamber, the
stroke H of the hammer piston and an impact coefficient q, if the hammer
mechanism has an impact frequency f during percussive operation:

wherein the parameter N is at least 4, po designates the ambient
pressure and κ the isentropic coefficient of gas in the pneumatic
chamber.

[0013]The maximum length of the pneumatic chamber is the distance of the
hammer piston from the flying mass, when the hammer piston is arranged in
its position away from the tool receptacle and the flying mass is
arranged adjacent to the impact surface. The maximum length is used as
the value to design and characterize the hammer mechanism. During
operation, the pneumatic chamber as a rule does not occupy the maximum
length at any point in time.

[0014]The impact coefficient q designates the ratio of the velocities of
the flying mass after the impact to before the impact. The impact
coefficient is determined essentially only by the masses and shapes of
the flying mass and the impact body.

[0015]One cycle of the flying mass in the hammer mechanism is made up of a
first phase with a movement from the minimum approach of the hammer
piston to the impact and a second phase with a movement from the impact
position to the next minimum approach of the hammer piston. The first
phase and the second phase are completed together within a period of
time, which is predetermined by the cycle duration of the movement of the
hammer piston. Due to the deceleration of the flying mass until the
momentary standstill, the duration of the second phase increases to the
detriment of the duration of the first phase. The flying mass overcomes
the distance between the minimum approach and the impact in a shorter
time, ergo, as desired, with a higher velocity.

[0016]The deceleration of the flying mass during the second phase takes
place if the dimensions of stroke and maximum length of the pneumatic
chamber are suitably selected. The pneumatic chamber is compressed at the
beginning of the second phase, because after the impact the hammer piston
is still moving in the impact direction or the flying mass is initially
moving with a greater velocity against the impact direction than the
hammer piston. In this connection, an increase in pressure is produced in
the pneumatic chamber, which decelerates the flying mass. The increase in
pressure is all the greater, the smaller the volume of the pneumatic
chamber or the greater the still remaining stroke movement of the hammer
piston is in the direction of the impact surface.

[0017]Based on hammer mechanisms that have been realized and numeric
simulations, it was recognized that with typical parameters with respect
to the mass of the flying mass, a diameter of the pneumatic chamber and
an impact frequency, in operation the cited ratio of 1.55 achieves an
increase in the impact energy based on a slow movement of the flying mass
in the second phase.

[0018]One embodiment of the invention provides that the stroke is selected
as a function of the maximum length of the pneumatic chamber such that
the flying mass changes the direction of movement at least once during
the movement between the impact surface and a following minimum approach
of the hammer piston. A ratio of less than 1.50 can be advantageous for
this. A change in the direction of movement during the second phase
produces a longer path, which the flying mass covers during a cycle. The
velocity of the flying mass is higher during the first phase, even taking
the basic condition of the predetermined period of time for a cycle into
consideration.

[0019]One embodiment provides that the stroke is selected as a function of
the maximum length of the pneumatic chamber such that the flying mass
touches the impact surface at least twice between two successive minimum
approaches of the hammer piston. A ratio of less than 1.40 can be
advantageous for this. The reversal of the direction of movement through
the second impact produces a high velocity of the flying mass at the end
of the second phase. The flying mass is thus able to closely approach the
hammer piston and afterward experiences a greater acceleration in the
direction of the impact surface due to the pneumatic chamber.

[0020]One embodiment provides that if the mass of the flying mass is
greater than 400 g, the length ratio is selected as less than 1.55 and if
the mass of the flying mass is less than 400 g, the length ratio is
selected as less than 1.40.

[0021]One embodiment provides that if a ratio of the mass of the snap die
to the mass of the flying mass is less than 1.2, the length ratio is
selected as less than 1.40.

[0022]The following description explains the invention on the basis of
exemplary embodiments and figures.

[0030]Unless otherwise indicated, the same or functionally equivalent
elements are identified by the same reference numbers in the figures.

[0031]FIG. 3 schematically depicts an electro-pneumatic chipping hammer 1
as an example of a percussive hand power tool, other examples not shown
are hammer drills and combination hammers, among others.

[0032]A drive train having a primary drive 3, a drive shaft 4 and a hammer
mechanism 5 is arranged in a machine housing. A gear 7 can be connected
between the primary drive 3 and the drive shaft 4. The primary drive 3 is
preferably an electric motor, e.g., a universal motor or a brushless
motor. The drive shaft 4 is rotated at rotational speeds in a range
between 1 Hz and 100 Hz, e.g., at 10 Hz to 60 Hz. The rotational movement
of the drive shaft 4 is transmitted by the hammer mechanism 5 in a
periodic impact movement along an impact axis 8. A tool held in a tool
holder 9 is driven from the chipping hammer 1 by periodic impacts along
the impact axis 8 in impact direction 99. Returning the tool to the
chipping hammer 1 against the impact direction 99 is accomplished by
pressing the chipping hammer 1 on a work piece.

[0033]FIG. 4 shows an exemplary structure of the hammer mechanism 5. The
hammer mechanism 5 has a hammer piston 12 and a flying mass 13, which are
moveable along the impact axis 8. In the depicted embodiment, the hammer
piston 12 and the flying mass are guided through a wall 11 of a piston
cylinder 10.

[0034]Positioned on a tool-side end of the piston cylinder 10 is a snap
die 20 in a snap die guide 21. A tool-facing end 22 is in contact with a
tool, which is held in the tool holder 9. An end 23 of the snap die 20
facing away from the tool projects out of the snap die guide 21 into the
interior space of the piston cylinder 10. In percussive operation, the
snap die 20 rests against an end 24 of the snap die guide 21 facing away
from the tool. In this position, the end 23 of the snap die 20 facing
away from the tool defines the position of the impact surface 27 of the
hammer mechanism 5.

[0035]The snap die 20 can be provided, as embodied, as an intermediary
between the flying mass 13 and a tool in the hammer mechanism 5. In
particular, this makes a design of the hammer mechanism 5 possible which
is independent of a mass of the tool being used. The snap die 20 for this
can be selected to be considerably heavier than the typical mass of the
tool.

[0036]In another embodiment, a snap die 20 is not provided. The flying
mass 13 impacts directly on an end surface of the tool. In this case, the
end surface forms the impact surface 27. The tool is inserted into the
tool receptacle 9 as far as possible in the direction of the hammer
mechanism 5. In this position, the tool defines the impact surface.

[0037]The hammer piston 12 is forced by the drive shaft 4 to make a
periodic movement along the impact axis 8 The drive shaft 4 is rotated
around its rotational axis 30 and in the process moves a wobble finger 31
arranged eccentrically to the rotational axis 30. The wobble finger 31 is
connected to the hammer piston 12 via a rod assembly 32. A stroke H of
the hammer piston 12 is defined as the distance between the two positions
at which the hammer piston 12 is closest and furthest away from the
impact surface 27. The stroke H of the hammer piston 12 is predetermined
by the distance 33 of the wobble finger 31 from the rotational axis 30
and corresponds approximately to double the crank radius 33 of the wobble
finger 31. The movement of the hammer piston 12 is periodic and,
depending upon the design of the eccentric drive 4, the movement is
sinusoidal or a good approximation of sinusoidal.

[0038]The hammer piston 12 and the flying mass 13 delimit a sealed
airtight chamber lying between them, the pneumatic chamber 19. A
cross-sectional area A of the pneumatic chamber 19 corresponds
approximately to a cross-sectional area of the flying mass 13 and of the
hammer piston 12. An airtight closure can be achieved, e.g., by sealing
rings 15, 16. The pneumatic chamber 19 has a maximum length L when the
hammer piston 12 is at a maximum distance from the impact surface 27 and
the flying mass 13 is adjacent to the impact surface 27.

[0039]A simple model of the trajectory of the flying mass 13 is explained
in the following on the basis of a conventional hammer mechanism and a
hammer mechanism 5 according to one embodiment. The model is used to
discover parameters of the hammer mechanism 5, with which the flying mass
13 is at least decelerated to a standstill between an impact on the
impact surface 27 and a following minimum distance from the hammer piston
12 or even changes its direction of movement.

[0040]FIG. 5 shows a trajectory 100 of the flying mass 13 for a
conventional, long hammer mechanism, plotted over the time t. The
trajectory 100 is determined by means of an ad-initio simulation. The
parameters of the hammer mechanism are: impact frequency f=14.5 Hz; mass
of the snap die m1=2.119 kg; mass of the flying mass m2=1.248
kg; stroke H=0.094 m; maximum length of the pneumatic chamber L=0.204 m;
cross-sectional area of the pneumatic chamber A=0.0034 m2; impact
coefficient q=0.25. The path curve 101 of the hammer piston 12 is also
plotted. FIG. 6 shows a trajectory 200 of the flying mass 13 for a short
hammer mechanism 5 according to one embodiment. The only parameter that
has been changed as compared with FIG. 5 is the maximum length L of the
pneumatic chamber: L=0.139 m.

[0041]The trajectory 100 of the long hammer mechanism can be divided into
two phases 102, 103 delimited by reversal points 104, 105 of the
trajectory 100. The first reversal point 104 is yielded by the minimum
distance of the flying mass 13 from the hammer piston 12. The second
reversal point 105 is produced by the impact of the flying mass 13 on the
impact surface 27.

[0042]The trajectory in the area of the first reversal point 104 can be
described by an impact of the flying mass 13 on the moved hammer piston
12. The effective mass of the hammer piston 12 is assumed to be infinite,
because the hammer piston 12 is rigidly connected to the drive. Typical
for a resonant excitation, the first reversal point 104 coincides with
the maximum velocity of the hammer piston 12. The velocity v1 of the
flying mass 13 after the first reversal point 104 is therefore
approximately ν1=2πHf+ν3, whereby v2 designates
the velocity prior to the first reversal point 104.

[0043]In the case of the impact of the flying mass 13 with the snap die 20
or the tool, the amount of the velocity v2 of the flying mass 13
after the impact is less than the velocity v1 prior to the impact,
because a portion of the kinetic energy of the flying mass 12 is
transferred to the snap die 20. The ratio (impact coefficient q) of the
velocities v2/v1 is specified by the mass m2 of the flying
mass 13, the mass mi of the snap die 20 and a form factor e of the impact
mates:

k = e m 2 - m 1 m 2 + m 1 . ##EQU00003##

The form factor e has values of 0 to 1; for short compact impact mates in
the vicinity of 1 and for more oblong structured impact mates in the
vicinity of 0. Sample values for the impact coefficient k are in the
range of 0.05 to 0.35. For example, the impact coefficient (q) can be
selected as 0.22, if a ratio m1/m2 of the mass (m1) of the
snap die to the mass (m2) of the flying mass (13) is greater than
1.2 and otherwise the impact coefficient (q) is selected as 0.12.

[0044]The volume V of the pneumatic chamber 19 changes during the first
phase 102 and the second phase 103. Consequently, the pressure p within
the pneumatic chamber 19 also changes. A force on the flying mass 13 is
produced because of the pressure difference between the environment
(approx. 1 bar) and the pressure p within the pneumatic chamber 19. The
flying mass 13 thus experiences an acceleration between the two reversal
points 104, 105, which increases or reduces its velocity v1,
v2.

[0045]The pressure p can be estimated by an adiabatic approximation, in
which (pV).sup.κ is constant, whereby κ (kappa) designates
the isentropic exponents (approximately 1.4 for air in the prevailing
pressure range of 0.5 bar to 10 bar) and V the volume of the pneumatic
chamber 19. It is assumed that a neutral volume V0 at which a
pressure p in the pneumatic chamber 19 corresponds approximately to the
normal pressure p0 of the environment (approximately 1 bar),
corresponds to half of the maximum length of the pneumatic chamber 19,
i.e., if the distance x of the flying mass 13 to the hammer piston 12 is
x=L/2.

[0046]In the case of the long hammer mechanism, the volume of the
pneumatic chamber 19 in the first and second phases 102, 103 changes only
negligibly compared to the neutral volume V0. This is caused to some
extent by the low stroke H, as compared to the maximum length L.
Correspondingly, only minimum deviations from the ambient pressure
p0 and low forces on the flying mass 13 are yielded. The effect of
the pneumatic chamber 19 on the movement of the flying mass 13 in the
case of the long hammer mechanism is insignificant. The velocity v1
during the first phase 102 and the velocity v2 during the second
phase 103 remain approximately constant.

[0047]It is approximately assumed that the flying mass 13 and the hammer
piston 12 touch each other at the first reversal point 104, at a distance
x=L-1/2H+b from the impact surface 27, wherein b is the length of the
flying mass 13. Under the basic condition that within one period, i.e.,
the period of time f-1, the distance L-1/2H must be covered once by
the flying mass 13 with the first velocity v1 and once at the second
velocity v2, yields the following for the first velocity:

v 1 = 2 π f H 1 - q . ##EQU00004##

[0048]In the case of the short hammer mechanism 5, the trajectory 200 also
has two reversal points 204, 205, which are produced by a minimum
approach of the hammer piston 12 and a subsequent impact on the impact
surface 27.

[0049]During the first phase 202, the flying mass 13 moves from the first
reversal point 204 to the second reversal point 205, in a similar manner
as with a long hammer mechanism. The velocity v1 is approximately
constant and is for instance ν1=2πHf+v3, whereby
ν3 is the velocity shortly before the first reversal point 204.
For an estimate of the velocity ν3=2f(α-1/2H), it can be
assumed that the movement from the impact surface 27 up to the first
reversal point 204 takes place approximately during a half period
(1/2f-1).

[0050]The second phase 203 of the short hammer mechanism 5 differs from
the second phase 103 of the long hammer mechanism. The velocity of the
flying mass 13 is decelerated to zero, in the depicted example the
movement of the flying mass 13 even reverses. The driving force for the
deceleration is produced by the strong coupling of the flying mass 13 to
the hammer piston 12 by means of the pneumatic chamber 19.

[0051]In the following, parameters of the hammer mechanism 5 are
estimated, at which the velocity v2 of the flying mass 13 is
decelerated at least to zero after the second reversal point 205.

[0052]The decelerating force is produced by the excess pressure
(p-p0) of the pneumatic chamber 19 with respect to the environment,
which excess pressure acts on the cross-sectional area A of the pneumatic
chamber 19. Due to the movement of the flying mass 13 in the direction of
the hammer piston 12, the volume V of the pneumatic chamber 19 also
diminishes and the excess pressure (p-p0) increases correspondingly.
The pressure change can be determined based on the adiabatic
approximation pV.sup.κ=p0V0.sup.κ.

[0053]The deceleration takes place typically at the latest within a
quarter of a period (T=1/4f-1) after the second reversal point 205.
During this period of time T, the hammer piston 12 moves slowly. A change
in the pressure p in the pneumatic chamber 19 is dominated during the
period of time T by the movement of the flying mass 13. After the period
of time T, the hammer piston 12 reaches a velocity which is clearly
greater than the velocity v2 of the flying mass 13. The relative
distance increases rapidly and is soon greater than 1/2L, which is why
the flying mass 13 is again accelerated in the direction of the hammer
piston 12.

[0054]During the period of time T, the position x1 of the hammer piston 12
is assumed to be approximately constantly equal to the minimum possible
distance to the impact surface 27 (x1=L-H). The volume of the pneumatic
chamber V during the period of time T is yielded as:
V=A(L-H-ν2t), wherein the velocity v2 is assumed to
calculate the volume V as constant.

[0055]The flying mass 13 stops when the integral of the decelerating force
over period of time T corresponds to the pulse of the flying mass 13,
i.e., ν2m2, after the second reversal point 204:

v 2 m 2 < ! ∫ 0 T A p 0 [ ( V
0 / V ) κ - 1 ] t . ##EQU00005##

[0056]Using the relationships described above and an expansion in series
according to time up to the first order produces the following with
T=(Nf)-1:

[0057]It is evident from the inequality that increasing the
cross-sectional area A, the stroke H and/or reducing the mass m2 of
the flying mass 13, the maximum length L of the pneumatic chamber 19, the
impact frequency f, tends to result in a hammer mechanism 5 in which the
movement of the flying mass 13 is decelerated to a standstill.

[0058]Parameter N is preferably greater than 4, based on the described
assumption that a deceleration takes place within a quarter period
T=1/4f-1.

[0059]It was stated in the introduction that selecting the impact
frequency f and the mass m2 of the flying mass 13 is subject to
narrow restrictions. The cross-sectional area A of the pneumatic chamber
19 is closely coupled with the shape and impact properties of the flying
mass 13. However, the external basic conditions can allow a largely free
selection of the maximum length L of the pneumatic chamber 19 and the
stroke H of the hammer piston 12.

[0060]For heavy hammer mechanisms 5 with a flying mass 13 of the mass
m2 greater than 400 g with otherwise typical parameters, such as a
large impact coefficient (q>0.2), the selection of the ratio of
maximum length L to the stroke H of: L/H<1.55 is suitable; and for
light hammer mechanisms 5 with the mass m2 less than 400 g, a
selection of the ratio: L/H<1.40 is suitable.

[0061]The hammer mechanism 5 is preferably operated resonantly such that
the first reversal point 204 and the greatest velocity of the hammer
piston 12 coincide, i.e., a difference of the respective points of time
of less than 2% of the cycle duration (T=f-1).

[0062]In the case of resonant operation, it is assumed based on
investigations of simulations and prototypes that a complete deceleration
takes place within a period of time T0=3/8f-1 after the first
reversal point 204. After the period of time T0, the velocity of the
hammer piston increases to 70% of its maximum value, whereby there is a
rapid decrease in the decelerating excess pressure to an accelerating
underpressure.

[0063]The flying mass 13 requires approximately a period of time from
1/8f-1 to 1/4f-1 for its movement to impact surface 27. The
deceleration can take place within a period of time of 1/8f-1 to
1/4f-1, which is why N is at least 4, preferably 6 or 8. For a
resonant operation, the parameters of the hammer mechanism 5 can be
determined in accordance with the above inequality with the selected N.

[0064]In another embodiment, the parameters of the hammer mechanism 5 are
selected such that the flying mass 13 in the hammer mechanism 5 touches
the impact surface 27 (point 206) a second time after the second reversal
point 205 before the flying mass 13 flies to the first reversal point
204. The lengthening of the trajectory of the flying mass 13 permits a
greater velocity while maintaining the impact frequency f.

[0065]So that the flying mass 13 returns to the impact surface 27, the
deceleration to a standstill must take place early on. Afterwards, an
excess pressure must still prevail for a sufficiently long period of time
in the pneumatic chamber 19 in order to accelerate the flying mass in the
direction of the impact surface 27. It was recognized from investigations
that this is achieved with a period of time T0 of less than
2/6f-1. The velocity of the hammer piston 12 achieves only 50% of
its maximum velocity within the period of time T0. The hammer
mechanism 5 can be designed in accordance with the above inequality,
wherein N is selected as greater than 5, preferably greater than 8 or 10.

[0066]The parameter N can be selected as greater than 8 for the two
impacts during a cycle of the flying mass.

[0067]The elements of a hammer mechanism can be arranged in diverse ways.
FIGS. 7 through 9 depict additional embodiments. The above outlined rules
for designing the hammer mechanism in FIG. 4 can also be applied to these
types of hammer mechanisms.

[0068]The foregoing disclosure has been set forth merely to illustrate the
invention and is not intended to be limiting. Since modifications of the
disclosed embodiments incorporating the spirit and substance of the
invention may occur to persons skilled in the art, the invention should
be construed to include everything within the scope of the appended
claims and equivalents thereof.